The term pasteurization is used herein in connection with the present invention in the general sense that the term is applied to other food products, e.g. pasteurized milk, in that the present pasteurized eggs are partially sterilized at temperatures which destroy objectionable microorganisms, without major changes in the functionality of the eggs. In this regard, food products are conventionally heated at temperatures and for times so as to sufficiently destroy pathogenic microorganisms, which may be contained in the food, so that the pasteurized food is safe for human consumption. In order to provide a pasteurized food safe for human consumption, it is not necessary that all pathogenic microorganisms in the food be destroyed, but it is necessary that those pathogenic microorganisms be reduced to such a low level that the organisms cannot produce illness in humans of usual health and condition. For example, fresh whole milk may contain virulent pathogenic microorganisms, most notably microorganisms which cause tuberculosis in humans, and during pasteurization of the milk, those pathogenic microorganisms are reduced to such low levels that the milk is safe for consumption by humans of ordinary health and condition. In the case of some microorganisms, however, usual pasteurization temperatures and times can completely destroy those microorganisms. Milk so pasteurized does not have major changes in the functionality thereof. The taste and texture of pasteurized milk is slightly changed, but those changes are not of practical significance to most consumers thereof.
Heat destruction of microorganisms in eggs has long been known in that the eggs were cooked sufficiently to effect destruction thereof. For example, when frying an egg, fried to a reasonable hardness, microorganism destruction will occur. Likewise, when boiling an egg to a hard-boiled state, heat destruction of microorganisms in the egg will occur. However, with these cooking processes, major changes in the functionality of the egg occurs, e.g. coagulation of the yolk and white, and, thus, this is not pasteurization in the usual sense, as explained above.
Recently, pasteurization of liquid chicken eggs (eggs out of the shell) has been commercially practiced. The process, very basically, involves heating liquid chicken eggs for short times at higher temperatures to reduce any pathogenic microorganisms therein such that the pasteurized liquid chicken eggs are safe for human consumption, while, at the same time, major changes in functionality do not occur. See, for example, U.S. Pat. No. 4,808,425.
However, the art has long since struggled with pasteurizing in-shell chicken eggs. While in-shell eggs may be heated sufficiently to destroy microorganisms, the art has not, at the same time, been able to substantially retain the functionality of the eggs. The functionality is determined by various tests, but a more basic test is that of the albumen functionality, which test measures the whipped volume, under standard conditions, of whipped liquid albumen, as measured in Haugh units.
In the case of liquid chicken eggs (not in the shell), by careful control of the time and temperature of heating the liquid eggs, usually with a short time, high temperature (HTST) apparatus, pasteurization can be achieved while retaining, at least substantially, the functionality of the eggs. This is particularly true when the liquid eggs are heated for pasteurization purposes in a very thin film, where the temperature and time of heating of the liquid eggs can be very carefully controlled.
In liquid eggs, the yolk may or may not be mixed with the albumen. As can be appreciated, however, with in-shell chicken eggs (also referred to as "shell eggs"), not only is the mass of the egg substantially different from the mass of a unit of thin film of liquid eggs, but the yolk is essentially centrally positioned in the shell. Accordingly, while the art has struggled for some time to carefully control temperatures and times for pasteurizing in-shell eggs, none of those efforts in the art have been successful in terms of both reducing pathogenic microorganisms found in chicken eggs to a level safe for human consumption while maintaining essentially the same functionality of the eggs as unpasteurized eggs. As a result, no commercial process for pasteurizing in-shell eggs and no commercial pasteurized in-shell eggs have been available.
The art has taken many different approaches in attempts to pasteurize in-shell eggs. See, for example, U.S. Pat. Nos. 1,163,873; 2,423,233; 2,673,160; and 3,658,558. The more prevalent approaches involve heating the in-shell eggs, usually in a water bath, for various times and at various temperatures, as specified by the various investigators in the art. These times and temperatures specified by the various investigators vary widely, and this is because all of those approaches involve a compromise either in the degree of safety achieved or in the quality of the functionality retained.
In this latter regard, if the in-shell egg is heated in a water bath, where the water bath temperature and time of heating are specified by the investigator, one of two results have generally occurred. The first result is that, when higher temperatures and longer times are specified, while the egg may be acceptably reduced in microorganism content, the functionality of the egg is also considerably reduced, such that the egg is no longer substitutable for unpasteurized eggs in either usual home cooking, e.g. frying, or in conventional baking recipes. The other result, when using lower temperatures of the water bath and shorter times, while the functionality of the egg is substantially maintained, the decrease in pathogenic microorganisms, which may be present in the eggs, is severely compromised, and the egg may be safer but not be safe for human consumption. While eggs processed according to this latter approach can be said to be safer to eat, in that there is some reduction of pathogenic microorganisms in the eggs, the eggs are not pasteurized in the sense as set forth above, i.e. that they are safe for consumption by humans of ordinary health and condition.
Faced with the above difficulties, that art searched for intermediate water bath temperatures and dwell times where functionality of the egg is preserved and microorganisms are substantially reduced. Unfortunately, these searches have generally resulted in the worst of both of the results noted above, i.e. both reduced functionality of the egg and still insufficient reduction in microorganisms, which result is less desirable than either of the two above-noted general results.
Accordingly, therefore, the art has been on the horns of a dilemma, i.e. if the times of dwell and temperatures of the water bath are high enough to substantially reduce the microorganism content of in-shell eggs, then the functionality of the eggs is substantially reduced, while if the times of dwell and temperatures of the water bath are sufficiently low as to substantially maintain the functionality of the eggs, the eggs are not sufficiently reduced in microorganism content so as to be pasteurized.
Pathogenic microorganisms are introduced into chicken eggs by two principal routes. Firstly, pathogens are introduced into the in-shell eggs from environmental contamination. This environmental contamination may occur through a variety of causes, but typically, infected chickens or mice in commercial egg-laying chicken houses deposit feces which contact the shell of a laid egg. Certain microorganisms, especially Salmonella, when in contact with the shell of the egg, can penetrate that shell, especially through small fissures or pores in the shell. That contamination is, therefore, from the outside of the shell into the egg, and the contamination remains, largely, in the albumen near the shell. This contamination can be very substantially reduced by the above-noted approaches of the prior art, since, when the egg is placed in a water bath heated to the temperatures suggested by the art, this is sufficient to heat the albumen near the shell and substantially destroy pathogens which may have penetrated the shell from environmental contamination. In this sense, the egg is, indeed, safer to eat.
The second route of contamination in the eggs is systemic, and this poses a far more difficult problem. Typically, feces of infected chickens or mice are ingested by the chicken during feeding, and that infection becomes systemic in the chicken. Certain organisms, very notably Salmonella enteritidis, enter the bloodstream of the chicken and pass, trans-ovarially, into the interior of the egg itself. Most especially, that systemic contamination occurs in the yolk of the egg, although that contamination can also easily extend into the albumen. In this type of contamination, the prior art approaches, as noted above, are ineffective toward substantially reducing microorganisms in the eggs, including the yolk, while at the same time maintaining the functionality of the eggs.
While many suggestions have been made in the prior art, principally, a water bath is heated to specified temperatures (although air, oil and the like heat transfer media have been suggested), and the in-shell eggs are then placed in that heated water bath and dwell therein for a specified length of time. It is generally assumed that the yolk temperature will come to equilibrium with the water bath temperature after a sufficiently long dwell time of the eggs.
Unfortunately, specifying the temperature of the water bath and the time of dwell of the eggs therein does not necessarily specify temperatures within the eggs, and especially the yolks. This is because eggs can vary in one or more of weight, size, shape, composition (e.g. relative size of yolk and air sack) and density, all of which affects the heat transfer properties of a particular egg in the water bath at the specified temperatures. Thus, when operating in water baths at specified temperatures within specified time ranges, the temperature within a particular egg, and especially the yolk, is entirely problematic, and, hence, the control of the prior art approaches toward pasteurizing eggs, especially in regard to yolk contamination, has been completely inadequate and more or less is a matter of chance--see, for example, WO 95/14388.
The specified temperatures of the water baths in the prior art vary considerably, with some investigators taking the approach of relatively low temperature baths, e.g. as low as about 100.degree. F., with long dwell periods of the eggs, while other investigators took the approach of high temperature baths, e.g. up to 160.degree. F., with relatively short dwell periods of the eggs, and others took an intermediate approach, e.g. 130.degree. F. to 140.degree. F., with intermediate dwell periods, e.g. 50 minutes. However, no matter which of these approaches is adopted, as explained above, the art simply has not found combinations of temperatures of water baths and times of dwell which will ensure eggs safe for human consumption, i.e. pasteurized eggs, including pasteurization of the yolks, while at the same time maintaining the functionality of the eggs. Accordingly, it would be a very substantial benefit to the art to provide a method for pasteurizing eggs where the eggs are not only pasteurized, i.e. safe for consumption by humans of ordinary health and condition, but which also assures that the functionality of the eggs is substantially retained.